Indoor sensors for subject localization and monitoring are currently carried out primarily by contact sensors such as transducers and/or optical image sensors. In most cases, such as for long-term health care, people are reluctant to wear contact sensors all the time, which may, at times, limit the applications of contact sensors mainly to emergency usage. Optic-based techniques, such as closed-circuit television (CCTV) and infrared camera, have been widely used in homeland security and surveillance. The recent advancement in the use of optical video such as KINECT® can provide an accurate depth image and has potential for uses in many applications. However, despite the great success they have achieved, optic-based techniques still suffer from several limitations. Although video can provide pictures with volume and detailed data, it is not good at revealing certain other information such as vital sign information (e.g. pulse, blood pressure, etc.) due to, among other reasons, the inadequate image resolution. Additionally, optical sensors can be easily blocked by obstacles between the target and sensors, hence require direct exposure and suffer from blind zone problems. On the contrary, non-contact microwave radar sensors have advantages over other alternatives. First, microwave radar is more sensitive in with respect to certain vital signs because it can provide millimeter or even sub-millimeter scale accuracy. Second, microwave sensors do not rely on light and can penetrate walls and other obstacles. Third, the Doppler and micro-Doppler characteristics reveal extra details of motion, and thus enable gesture recognition.
However, there are still limitations for current microwave radar sensor such that it cannot fully handle real time individual life activities. The greatest challenge for the microwave radar sensor is how to provide sufficient range detection and displacement monitoring accuracy at a low cost. There are several mainstream radar architectures that that is found in literature, i.e., Doppler (interferometry) radars, impulse-radio ultra-wideband (IR-UWB) radars, frequency modulated continuous wave (FMCW) radar and stepped frequency modulated continuous wave (SFCW) radar. Doppler radars operate based on single tone continuous wave to obtain phase history. They have been widely used because of their high precision in displacement measurement. However, they cannot detect range information with sufficient accuracy. Their inability to spatially distinguish multiple targets limits their applications mainly to indoor vital sign monitoring and gesture recognition. IR-UWB, FMCW and SFCW radars are capable of providing range information. However, their range resolution is highly dependent on the bandwidth transmitted, forcing most of the systems to work at high frequencies for the resolution required by life activities monitoring. Great efforts must be made to overcome the problems associated with high frequency and wide bandwidth (e.g., linearity and frequency drift problems), not to mention the high cost, complexity, and high attenuation it suffers.